Currently, if users want to resize each portalchildren, they need to specify sizable="true" on each panel within a portallayout. There are several issues:

• this is a portallyout / portalchildren's feature, but users have to specify an attribute on a panel instead of a portallyout.
• Users have to specify the attribute on each panel which is unnecessary since it doesn't allow partially sizable panels.

Therefore, we should provide a way to enable resizing on a portal layout, e.g.:
<portallayout sizable="true">
or
<portallayout mode="grid">

Then users can resize each panel in each portalchildren like splitpane.